The present invention relates to a playback apparatus of a disc-like recording medium on which data have been recorded according to the constant linear velocity (CLV) scheme. For example, the disc-like recording medium is what is called a compact disc (CD).
Among memories dedicated to reproduction is a CD-ROM (compact disc-read only memory), which is played back while the disc rotation speed is controlled according to the constant linear velocity (hereinafter referred to as CLV) scheme. Therefore, in a disc playback apparatus for playing back CD-ROMs, to realize the above CLV control, the disc rotation speed is varied in accordance with the position on a disc (track position or radial position) which position is being subjected to reproduction by an optical head, i.e., an optical pickup.
In recent years, disc playback apparatuses are common that play back a CD-ROM at a speed two times (2-fold-speed playback), four times (4-fold-speed playback), or six times (6-fold-speed playback) the normal playback speed, or at other accelerated speeds. Apparatuses capable of playing back a CD-ROM at a speed even eight times the normal playback speed (8-fold-speed playback) are now being put into the market. The "normal playback speed" as used above is equal to the playback speed of the compact disc (CD), i.e., 1.2-1.4 m/s. In the following description, the 2-fold-speed playback, 4-fold-speed playback, 6-fold-speed playback, 8-fold-speed playback, etc. are collectively referred to simply as "accelerated speed playback."
The above disc playback apparatuses perform the CLV control even in playing back a CD-ROM at an accelerated speed as mentioned above. As a result, in an accelerated speed playback, a difference between a disc rotation speed at the disc innermost track and that at the outermost track is larger than in a normal speed playback. The difference is particularly large in an 8-fold-speed playback.
FIG. 1A shows a relationship between the radial reproduction position on a disc and the disc rotation speed, that is, the number of revolutions per minute of a spindle motor for driving, i.e., rotating the disc, in an 8-fold-speed playback. In the case of playing back a CD-ROM at the normal playback speed, it is prescribed that the linear velocity should be within the range of 1.2 to 1.4 m/s. In FIG. 1A, a solid line and a broken line represent relationships between the reproduction position (track position) and the motor rotation speed in 8-fold-speed playbacks corresponding to the normal playback speed linear velocities 1.4 m/s and 1.2 m/s; respectively. For example, in the case of an 8-fold-speed playback corresponding to the normal playback speed linear velocity 1.4 m/s, the number of motor revolutions per minute is 4,240 rpm at the disc innermost track, gradually decreases toward the disc outermost track, and is 1,867 rpm at the outermost track. In the case of an 8-fold-speed playback corresponding to the normal playback speed linear velocity 1.2 m/s, the number of motor revolutions per minute is 3,634 rpm at the disc innermost track, gradually decreases toward the disc outermost track, and is 1,600 rpm at the outermost track. Thus, as seen from FIG. 1A, the difference in the number of motor revolutions per minute between the disc innermost and outermost tracks is very large.
FIG. 1B shows a relationship between the radial position on a disc and the transfer rate of data that is output from the disc playback apparatus when a CD-ROM is played back at the 8-fold speed as mentioned above. As seen from FIG. 1B, when a CD-ROM is played back at the 8-fold speed under the CLV control, the data transfer rate is constant, that is, data transfer rates at the disc innermost and outermost tracks are the same. The transfer rate of the 8-fold-speed playback under consideration is 1,200 KB/s (kilobytes per second).
In what is called a full-stroke access in which a playback is performed by moving the optical pickup rapidly from the disc innermost track to the outermost track or vise versa, it is necessary to rapidly decrease the disc rotation speed from the one at the innermost track to the one at the outermost track, or rapidly increase the disc rotation speed from the one at the outermost track to the one at the innermost track. To this end, the spindle motor for rotating a CD-ROM is required to be capable of generating large torque. In particular, to reduce the time required for a full-stroke access, very large torque is needed. Spindle motors that can produce such large torque are expensive, and the mechanical parts of a CD-ROM drive should be ones capable of withstanding such large torque. Further, since the spindle motor is accelerated or decelerated at each access, large power is consumed for driving of the spindle motor.
As described above, to perform an 8-fold-speed playback, the disc rotation speed should be made maximum at the disc innermost track and the number of motor revolutions per minute at the disc innermost track amounts to 4,240 rpm at the maximum, possibly resulting in very large vibration due to surface shakes, eccentricity, etc. of a disc. Therefore, the mechanical parts of the CD-ROM drive should be highly accurate ones capable of minimizing surface shakes and eccentricity of a disc as well as be highly strong.